1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium improved in the running endurance.
2. Description of Prior Arts
Recently, a demand for higher dense recording has increased in a magnetic recording medium (also referred to hereinafter as a magnetic tape) such as an audio tape, a video tape, or a recording medium employed in a computer system, and hence the size of a ferromagnetic powder conventionally employed in the magnetic tape such as .gamma.-iron oxide or Co-containing .gamma.-iron oxide is minimized. Further, a magnetic recording medium using a ferromagnetic metal powder composed of non-oxide type metal or alloy such as a metal audio tape and a metal video tape has been practically employed.
At present, as the magnetic recording medium such as an audio tape, a video tape, or a recording medium for a computer system, a magnetic tape having a structure comprising a nonmagnetic support, a magnetic recording layer provided on one surface of the support and a back layer provided on another surface of the support for enhancement in the running property of the tape is generally employed. In the magnetic recording medium having such structure, the surface of the magnetic recording layer is especially smoothened to enhance the sensitivity (particularly the output level in the high-frequency region). Further, the magnetic tape of recent days is prepared in the form of a thin film having whole thickness of not larger than approx. 20 .mu.m, by making each thickness of the support, magnetic recording layer and back layer smaller.
The back layer of the magnetic recording medium such as a magnetic tape is in the form of a thin film generally comprising carbon black and an inorganic filler powder both dispersed in a binder, and contributes to the improvement of running property of the magnetic recording tape as described hereinbefore.
However, the present inventors have found that the drop-out phenomenon is prominently brought about in the running procedure of the magnetic tape and the surface of the recording layer is markedly damaged (or scratched) after the repeated running of the tape by making the thickness of the back layer smaller. As a result of further studies by the inventors, it has been confirmed that the above-mentioned increase of drop-out phenomenon and occurrence of damages (or scratches) on the surface of the magnetic recording layer after repeated running are caused by a great number of inorganic filler particles protruded on the surface of the back layer. In more detail, a magnetic tape such as an audio tape or a video tape is stored in such a condition that the tape is wound up around a hub, and the tape is automatically wound around a hub on another side after the running procedure is complete. In the winding stage and the storage in the wound state, the magnetic recording layer and the back layer are placed in close contact with each other under pressure. Hence, if a great number of inorganic filler particles are protruded on the surface of the back layer, the surface of the magnetic recording layer is damaged (or scratched) by the protruding tips of the filler particles, and thereby occurrence of drop-out phenomenon increases in the running procedure of the magnetic tape.